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Old 07-25-2009, 05:01 PM   #20 (permalink)
aerohead
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thinks

Quote:
Originally Posted by theycallmeebryan View Post
I would like to hear what aerohead thinks about what i have talked about so far.

theycallmeebryan,good feedback and good eyes.Perhaps you can help out on Marchaj's table for sections.My thought was that at the Reynolds number listed,the relationship between drag and fineness ratios/thickness, would be be maintained across the Reynolds number spectrum,as all airfoils perform as a function of Reynolds number ( all else being equal )and that skin friction is the primary source of drag for sections up to stall,where profile drag would dominate.From my fluids text,for sections in 2-dimensional flow,thickness ratios below 2:1 are dominated by profile drag.Above 4:1 skin friction is the dominant player.--------------------------------------- With respect to the 67-series wing section,I have only data for the 67,1-215 section.It shows a drag minimum at Re 6-million-------------------- Here's the freaky thing about wings.In my Abbott and von Doenhoff' Theory of Wing Sections,on page 28,Chapter-1 it says that"section data are not applicable to wings of low aspect ratio.In fact,an entirely different theory applies to wings of very low aspect ratio." It cites Reference 61,Jones,Robert T.:Properties of Low-aspect Ratio Pointed Wings at Speeds below and above the Speed of Sound.NACA TN No. 1032,1946,as source of data for this situation.------------------- The other thing is that for anything other than zero crosswind,a wing,standing on it's head is experiencing a change in angle of attack,lift is induced,which creates a pitching-moment,and also,friction drag increases due to velocity increase due to lift,and pretty soon your Cd min ( @ zero "angle-of-attack") is right out the window.----------------- Sylph was designed around a NACA 66-section( P-51 Mustang ) and while it tunneled out in 1/4-scale with Cd 0.109 it's still 11% dirtier than GM's pumpkin-seed Sunraycer.--------------------------- And evidently,some wings are chosen for "flight Reynolds number" which would not be experienced in a car until over 200-mph.---------------------- I know BicycleBob and others have a much deeper grounding in struts and wing sections,and the station positions make it great to produce these forms,and I'm not in opposition to their use,however,until I dig a little deeper,it seems like we have to temper our enthusiasm for them when considering the context of their design and limitations imposed by ground-effect,crosswinds,extremely low aspect ratio,spanwise circulation,etc..
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